1. Technical Field
The present invention relates to a power amplifier circuit for an audio signal and an audio device using the same and, particularly, to a power amplifier circuit capable of reducing power consumption in a Balanced Transformer Less (BTL) type output circuit of an audio device, particularly, a portable audio device, such as a radio receiver, a cassette tape player, video tape recorder, a video camera or a component stereo device, which generates an audio signal.
2. Background Art
FIG. 9(a) shows a schematic block circuit diagram of a signal reproducing system in a conventional portable cassette tape player as an example of an audio device using the BTL output circuit. 1 depicts a read head, 2 a signal reproduction processing circuit including a head amplifier and an equalizer circuit, etc., 3 an output stage amplifier of positive phase side (non-inverted output side), 4 a loud speaker as a load and 5 an output stage amplifier of negative phase side (inverted output side).
During reproduction, the audio signal recorded on a tape (not shown) is read out therefrom through the read head 1 as a read signal A which is an input audio signal. The read signal A is inputted to the signal reproduction processing circuit 2 in which a high frequency bias component applied during recording is removed and, after equalization processing, outputted as an audio signal B. The thus reproduced audio signal B is finally amplified by the output stage amplifiers 3 and 5 in which output signals C and C* are produced and the loud speaker 4 is driven by these outputs. As a result, a reproducted sound is generated by the loud speaker 4.
It is usual that the transistor amplifiers 3 and 5 include in their input stages input stage amplifiers 3a and 5a for producing a pair of signals. The audio signal B is amplified by the input stage amplifier 3a and made into a pair of signals whose phases are different from each other by 180.degree.. These signals are amplified by push-pull transistors Q1 and Q2 which constitute an output stage amplifier to provide the amplified output signal C. The audio signal B is inverted and amplified by the input stage amplifier 5a, amplified similarly by push-pull transistors Q3 and Q4 and power-amplified to provide the output signal C*.
When describing in detail the power amplification in the output stage amplifier 3 as an example, the voltage of a power source line Vcc which feeds the output stage amplifier 3 in accordance to the input signal B is lowered by the transistor Q1 to a voltage level of the output signal C. In other words, the output signal C is produced as a result of voltage drop by an amount of an internal impedance of the transistors Q1 and is changed according to a waveform of the audio signal B. In this case, the transistors Q1 handles a voltage difference between the power source line voltage Vcc and the voltage of the output signal C. As a result, the transistor Q1 consumes a power corresponding to the voltage difference.
Although the output stage amplifier is shown in the figure as comprising a simple circuit including the output transistors Q1 and Q2, a peripheral circuit including a drive circuit, etc., may be included in a practical circuit construction. The above matter is the same for the output stage amplifier 5. Particularly, in the case of the BTL circuit, although the input stage amplifiers 3a and 5a are constructed by differential amplifiers and there is provided negative feedback from output terminals of the amplifiers 3 and 5 to the inverted input side of the input differential amplifiers 3a and 5a to which a reference voltage (corresponding to Vcc/2 where a Vcc is power source voltage) is supplied, such is omitted in the figure since it has no direct relation to the present invention.
Describing the operation of the BTL output stage amplifiers 3 and 5 in detail, when a voltage value of the audio signal B is higher than the reference voltage (Vcc/2), the transistor Q1 on the side of the power source is made active by the output of the input stage amplifier 3a and the transistor Q2 on the ground side is cut off. Further, the transistor Q3 on the side of the power source is cut off and the transistor Q4 on the ground side is made active by the output of the input stage amplifier 5a. And, current corresponding to the voltage value of the audio signal B flows from the power source line Vcc through the transistor Q1, the loud speaker 4 and the transistor Q4 to ground.
When the voltage value of the audio signal B is lower than the reference voltage, the ON and OFF operations of the transistors are reversed with respect to those mentioned above and current corresponding to the voltage value of the audio signal B flows from the power source line Vcc through the transistor Q3, the loud speaker 4 and the transistor Q2 to ground.
When the voltage value of the audio signal B is equal to the reference voltage, the respective transistors are in an OFF state. In this case, due to the negative feedback to the input stage amplifiers 3a and 5a, output terminals of the amplifiers 3 and 5 become Vcc/2, respectively.
Power consumed by the respective transistors Q1, Q2, Q3 and Q4 when the pair of output stage amplifiers 3 and 5 which operate in negative phase each other are provided and are operated in BTL operation in this manner is shown by hatching in FIG. 9(b). In this figure, power consumption of the respective transistors is shown by areas hatched with lines in different directions.
Power caused by the voltage drop in the output transistors shown by the hatching is dissipated by the power amplifier transistor as heat. Therefore, transistors whose power loss is large are required. Since a large power is consumed there, power efficiency in the case of the production of the output signals C and C* by the BTL circuit is low.
This fact is a problem, in particular for a portable audio device which is operated by a battery of limited capacity since an operating time thereof depends upon the efficient utilization of power. Further, for such a device, it is very important, as a commercial product, that it is operable for a long period of time. Therefore, power consumption of the device should be as small as possible.
For fulfilling these demands the present assignee has already filed two U.S. patent applications of U.S. Ser. No. 08/199,890 and U.S. Ser. No. 08/203,307 for switching regulator type power amplifier circuits.